In medical imaging so-called “hybrid modalities” are becoming increasingly important, for example PET-CT, SPECT-CT, PET-MRI and SPECT-MRI. The meanings of these abbreviations are as follows:
PET: Positron Emission Tomography
CT: Computed Tomography (e.g. X-ray)
SPECT: Single Photon Emission Computed Tomography
MRI: Magnetic Resonance Imaging
The advantage of these combinations is the combination of a modality with a high local resolution (especially MRI or CT) with a modality with high sensitivity (especially SPECT or PET) so that anatomical images may be combined with biologic function images for a more complete clinical image.
In the case of MRI, locating an array of imaging detectors inside the bore of a MRI system presents to the designer many technical challenges, which are exacerbated not only by spatial constraints, magnetic and gradient fields, but also by ohmic and eddy current heating. In general, the magnetic field strength and spatial constraints of a MRI system limit optical sensor selection to solid-state types, such as avalanche photo diodes (APDs) and silicon photo multipliers (SiPMs). However, most conventional PET and PET/CT detectors rely on photo multiplier tubes (PMTs) as the primary optical sensing element since they are generally insensitive to temperature fluctuations since their photon conversion elements are incased within an evacuated tube. Unfortunately, most PMTs are highly susceptible to changes in magnetic fields. Further, most conventional PMTs for PET and PET/CT detectors are relatively large (>100 mm). Accordingly, PMTs are generally impractical for use in the spatially restricted, magnetic environment of an MRI system.
In contrast, solid state photosensors, such as APDs and SiPMs, are relatively small in height (1-2 mm) and are typically unaffected by the magnetic fields of an MRI system. However, the gain of such sensors can significantly fluctuate in response to temperature fluctuations of the ambient environment. Therefore, when APDs and SiPMs are used to form an optical sensing element for a PET detector, such gain fluctuations can be very detrimental to the operation of the PET detector, as it generally requires long-term stability in the optical sensing element.
SiPMs have a much higher gain than APDs (about 106, compared with APD of about 200) and a time-of-flight detection capability, and may largely replace APDs in the future. However, as a solid state photosensor, a SiPM is also sensitive to its bias voltage as well as its body temperature. Therefore, in order to successfully use SiPMs, more accurate temperature compensation and higher stability bias voltage regulation must occur.